The present invention relates to a coloring method and a color-forming material. More particularly, the present invention relates to a coloring method and color-forming material providing a colored image or developed color image excellent in chemical and light resistance.
As a method for coloring papers, films, sheets, shaped articles, and other articles, there has ordinarily been adopted a method in printing, patterning or solid coloration is carried out by using colorants such as inks, dyes, pigments, and paints. As another coloring method, there is known a method in which a color-forming property is given in advance to an article to be colored and an energy or auxiliary substance necessary for the color formation is applied to the article from the outside to effect the color formation. With recent development of the information-oriented society, the latter method for effecting the color formation on an article to be colored has made rapid progress as a means for converting various information to visible images and recording them. For example, there are known a heat-sensitive color-forming method in which a chemically color-forming layer is formed on the surface of a support and the color formation and recording are effected with a heat energy, and a pressure-sensitive color-forming method in which the color formation and recording are effected with a pressure energy. These color-forming methods are not only applied in the field of the information industries but widely utilized for coloration and patterning of various daily necessities and industrial products.
As a typical instance of the color former for imparting a color-forming property to the surface of an article to be colored, there is known a two-component type color former comprising a leuco dye such as a phthalide, fluoran, or phenothiazine and an acidic substance. Since an excellent color-forming property is manifested by the color-forming method using such a leuco dye, this method is widely put into practical use in the fields of heat-sensitive recording and pressure-sensitive recording. However, most colored images obtained by this method, for example, colored images formed with the use of Crystal Violet Lactone, are defective in that the light resistance is poor and color fading is caused when the colored images are exposed to light for a relatively short time. Moreover, colored images obtained by the method using leuco dyes are poor in chemical resistance and the formed colors are readily extinguished by chemicals contained in foods, cosmetics, and plasticizers contained in vinyl chloride resins and rubber erasers.